One of the problems which the invention seeks to solve relates to the adaptation of the characteristics of an M-JPEG2000 compressed stream to the constraints of a transmission channel with limited throughput, typically a radio transmission channel. A compressed stream delivered by a JPEG2000 coder possesses the capacity to group together, in one and the same stream, several versions of the same image with various resolutions but also with various levels of quality. One conventionally speaks of scalability to designate the capacity of a source coder to produce, in a single stream, several versions of the same image that are accessible to the user. In particular, a stream compressed according to the M-JPEG2000 standard is a stream with multiple spatial resolutions. It is conventionally composed of sub-bands obtained subsequent to the application of a wavelet transform to an image. Each sub-band represents the original image at a lower resolution. Knowing the maximum throughput available on the transmission link, it is possible to adapt the compressed stream by deleting certain sub-bands so as to comply with the constrained throughput. The compressed stream resulting from this adaptation will culminate, after decoding, in an image of lower resolution. A problem to be solved in this context relates to the selection of the sub-bands to be transmitted so as to minimize the distortion induced on the final image. The throughput adaptation can also be performed, conversely, by increasing the total throughput of the stream by applying an error-correcting code to one or more sub-bands so as to minimize the distortion resulting from the impact of transmission errors on these sub-bands.
Moreover, M-JPEG decoders make it possible to implement error masking techniques relying on the temporal correlation existing between several successive images. Thus, certain parts of a decoded image may be reconstructed on the basis of the information present in one or more earlier images in the stream. The temporal redundancy existing between certain successive images may be utilized upstream so as to reconstruct with the maximum resolution an image received with a lower resolution.
The applicant's French patent application published under the number FR2944935 is known. It describes a method for estimating the throughput and the distortion of an image compressed by a source coder of JPEG2000 type. This method makes it possible to estimate the distortion induced by the impact of transmission errors on certain parts of a JPEG 2000 compressed stream. On the other hand, it does not take into account the capacity of an M-JPEG 2000 decoder to mask the errors resulting from the entire loss of a sub-band on the basis of compensation of motion between a key image and the image received.